Stabilized thermoplastic polymer compositions comprising a C-nitroso compound

ABSTRACT

Thermoplastic polymers are stabilized by mixing therewith, under polymer processing conditions such that nitroxyl radicals are formed, a nitrogen stabilizers therefor, which is (a) a C-nitroso-containing compound (b) an N-nitroso-containing compound, or (c) a nitrone-containing compound.

This invention relates to the stabilization of thermoplastic polymercompositions.

Most thermoplastic polymers are subject to oxidative deteriorationduring processing or in service. This deterioration can lead to loss ofproperties by the polymers, e.g. embrittlement or discolouration, whichcan be undesirable. Most particularly thermoplastic polymers are subjectto deterioration when, in service, they are in an outdoor environment orotherwise subjected to U.V. radiation. In addition deterioration canoccur, e.g. as a result of oxidation, when, in service, the polymers aresubjected to high temperature. Also the high shearing forces andtemperatures used in polymer processing often result in deterioration.

It is well known to incorporate so-called stabilizers into thermoplasticpolymer compositions to prevent or retard such deterioration processes.Such stabilizers, according to the manner in which they act, are U.V.stabilizers, thermal antioxidants or melt stabilizers. Amongst thecommercially available U.V. stabilizers are the compounds of theformulae ##STR1## [bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate] and##STR2## which are sold under the names Tinuvin 770 (ex Ciba-Geigy AG)and Cyasorb UV 531 (ex American Cyanamid) respectively. Suchconventional stabilizers, in particular Tinuvin 770, are often expensiveto prepare. In order to ensure that a particular polymer composition hasthe required stability characteristics, it is often necessary to use amixture of stabilizers. For example there may be added to the polymerboth a U.V. stabilizer and a melt stabilizer to protect the polymer fromdeterioration both during the processing of it and during its service.This of course tends further to add to cost.

The mechanism of the stabilization is not fully understood and clearlydifferent stabilizers can act in different ways. For example themechanism by which Tinuvin 770 stabilizes thermoplastic polymers isdifferent from that by which Cyasorb UV 531 stabilizes them. Recentstudies (Bagheri, Chakraborty & Scott, Polymer Degradation and Stability4, (1982), 1-16) on the use of hindered piperidines, of which Tinuvin770 is an example, attribute stabilizing effectiveness to the oxidationof the hindered piperidines to nitroxyl radicals during the polymerprocessing and subsequently when the thermoplastic polymer mixture isexposed to U.V. radiation. It is the presence of these nitroxyl radicalswhich is believed to give rise to the stabilization.

It has now been found that thermoplastic polymers, in particularpolyolefins, e.g. polyethylene and polypropylene, and polyvinylchloride, may be stabilized against oxidative deterioration by mixingtherewith a C-nitroso-containing compound, an N-nitroso-containingcompound or a nitrone compound under polymer processing conditions suchthat there are formed nitroxyl radicals.

According to the present invention, there is provided a process forpreparing a stabilized thermoplastic polymer composition which processcomprises mixing the thermoplastic polymer with a nitrogen-containingstabilizer, which is

(a) a C-nitroso-containing compound,

(b) an N-nitroso-containing compound, or

(c) a nitrone-containing compound,

under polymer processing conditions such that nitroxyl radicals areformed.

It is important that the compositions prepared according to the presentinvention are formed under polymer processing conditions such thatnitroxyl radicals are formed. The required conditions are conditions inwhich the polymer is subjected to high shear in which polymer chainsbecome severed resulting in the mechanochemical formation of hydrocarbylradicals. Mere mixing without shear, e.g. of polymer and stabilizersolutions, will not provide the U.V. stabilization properties of thepresent invention. While we do not wish in any way to be bound by thetheory, it is believed that the macrohydrocarbyl radicals and relatedmechanochemical radicals formed from the polymer in the processing stepreact with the stabilizer compounds according to the present inventionto form the nitroxyl radicals. In particular it has been observed thatstabilization obtained with the compounds according to the presentinvention is dependant upon the concentration of nitroxyl radicals inthe composition after processing. In the case of the C-nitroso compoundsand the nitrone compounds, these are believed to react directly with thehydrocarbyl compounds in a radical trapping reaction to form thenitroxyl radicals. In the case of the N-nitroso compounds, it isbelieved these dissociate and react further to give two different kindsof nitroxyl radical.

The polymer processing conditions necessary for nitroxyl radicalformation according to the present invention depend, in particular, onthe thermoplastic polymer used. Generally speaking the polymer andstabilizer are mixed together under high shear at above 150° C., usually160° to 350° C., and preferably 180° to 280° C. in a mixerconventionally used for polymer processing. During the mixing shear isat its highest initially and decreases as the polymer is broken up. Itis thus initially that the polymer chains are severed with hydrocarbylradical formation. It is important according to the present inventionthat the stabilizer should be present during this high shear to obtainthe good results according to the invention. Processing time alsodepends on the ingredients being used but will generally be 10 to 20minutes.

It has been found that the C-nitroso compounds (i.e. compoundscontaining at least one C--N═O group) have good U.V. stabilizingactivity for, i.e. they prevent or retard the photo-oxidativeembrittlement of, thermoplastic polymers. Also the C-nitroso compoundshave melt stabilizing activity. The C-nitroso compounds may inparticular be aliphatic or aromatic compounds with nitroso-alkanes beingpreferred. Generally speaking aromatic nitroso compounds are lesseffective as U.V. light stabilizers but are more effective than theiralkyl analogues as thermal antioxidants.

C-nitroso compounds according to the present invention include those ofthe general formula I:

    R--N═O                                                 (I)

wherein R represents an unsubstituted or substituted alkyl group,preferably a tertiary alkyl group (e.g. tertiary butyl or octyl, or agroup C(CH₂ OR')₃ or a group R_(x) C(CH₂ OR')_(3-x) wherein x is 1,2 or3, R is as defined below and R' represents a hydrogen atom or anunsubstituted or substituted alkyl or acyl group and in which each R'may be the same or different), and R may be an unsubstituted orsubstituted (e.g. alkyl-, chloro-, hydroxy-, carboxyl-, cyano-, nitro-,or dimethylamino-substituted) phenyl or naphthyl group (e.g. phenyl,tetramethyl-phenyl, pentamethyl-phenyl, trichloro-phenyl,hydroxy-phenyl, di-tert-butyl-hydroxy-phenyl, dimethylamino-phenyl andhydroxy-naphthyl) or a heterocyclic aromatic or reduced heterocyclic(e.g. piperidinyl) group.

Particularly preferred C-nitroso compounds are the tertiary alkylnitrosos, particularly 2-nitroso-2-methylpropane and2-nitroso-2,4,4-trimethylpentane. These compounds are highly effectiveas U.V. stabilizers and act as melt stabilizers. At the same time thecompounds are much cheaper to produce than conventional commerciallyavailable stabilizers.

It has also been found that N-nitroso compounds (b) act as U.V.stabilizers to prevent or retard the photo-oxidative embrittlement ofthermoplastic polymers. These compounds are also generally effective asmelt stabilizers. The N-atom of the N-nitroso group can be attached toaliphatic, cycloaliphatic, aromatic and araliphatic groups. Howeverpreferably the nitrogen atom is attached to two alkyl groups to give abranched structure described generally as: ##STR3## wherein R" and R"'are branched chain hydrocarbon radicals, preferably tertiary. Mostpreferably the nitrogen atom forms part of a saturated 6-membered ring,which may contain other N-hetero-atoms. Particularly preferred suchcompounds are ##STR4## wherein each R^(1V) group represents hydrogen oralkyl.

The first of these compounds is a novel compound and corresponds to thebis nitrosamine derived from the commercially available Tinuvin 770product. It has almost twice the effectiveness as a U.V. stabilizer ofTinuvin 770 and also is an effective melt stabilizer and thus it is apreferred compound according to the invention. Thus, unlike Tinuvin 770itself, which is not an effective melt stabilizer, it does not require aseparate melt stabilizer to be added to the system.

It has further been found that nitrone compounds (c), in particularaldonitrones, preferably N-phenyl and N-tert.alkyl aldonitrones, whenused in thermoplastic polymers are stabilizers. These compounds havegood melt stabilizing activity and have thermal oxidative stabilizingeffect. Nitrone compounds are generally weak U.V. stabilizers when usedalone.

Preferred nitrones are those of the general formula II ##STR5## whereinR₁ represents an alkyl or substituted alkyl, preferably tertiary alkyl(e.g. tert. butyl), group or an aromatic or substituted aromatic,preferably phenyl, group, R₂ represents a hydrogen atom or an alkyl oraryl group, and X and Z may each represent a hydrogen atom or a loweralkyl, e.g. methyl, group and Y represents a hydrogen atom or a hydroxyor alkoxy, e.g. methoxy, group. Most preferably there is used abenzaldonitrone containing a partially hindered 4-hydroxy group on thephenyl ring.

In choosing the stabilizer to use according to the present invention,regard has to be taken of the physical properties of the stabilizer.Thus for example in many applications the use of a stabilizer which ishighly coloured needs to be avoided. Also for example stabilizers whichare lost to the system e.g. by being volatile under the processingconditions should be avoided.

The stabilizers according to the present invention are in general knowncompounds and may be obtained by methods known per se. For example theymay be obtained as follows:

Nitroso alkanes may be prepared by oxidation of the corresponding amineswith hydrogen peroxide by the methods of Stowell (J. Org. Chem., 36,3055 (1971)).

Aromatic nitroso compounds may be prepared by oxidation of thecorresponding amine with hydrogen peroxide by the method of Richard andBayer (J. Am. Chem. Soc., 82, 3455 (1960)).

Nitrones may be prepared by reacting a hydroxylamine with thecorresponding aldehyde by the reaction of Beckman (Chem. Ber., 27, 1894(1958)).

Nitrosamines may be prepared by the method of Vogel (Prac. Org. Chem.,Longman (1967)).

As mentioned above however the bis nitrosamine derived from thecommercially available Tinuvin 770[bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate] is a novel compound.This compound can be obtained from the sebacate by reaction with sodiumnitrite in the presence of acid.

The thermoplastic polymers of the stabilized compositions according tothe present invention are generally saturated thermoplastic polymers, inparticular polyolefins, e.g. polyethylene and polypropylene, andpolyvinyl chloride. However there may generally be used other organicthermoplastic polymers with carbon-carbon bonds including polyesters,polyamides and rubber modified plastics such as ABS and rubbers whichare used as thermoplastics without vulcanisation or cross-linking suchas ethylene/propylene copolymers and styrene butadiene block copolymers.

One interesting aspect of the present invention is the use ofstabilizers according to the invention in conjunction with otherstabilizers. As noted above the C-nitroso and the N-nitroso compoundsact in particular as U.V. stabilizers and in addition have meltstabilizing activity. The nitrones have in particular good meltstabilizing effect and have thermal antioxidant activity. By using otherstabilizers in conjunction with the stabilizers according to theinvention, stabilizing effect may be broadened and increased.Particularly interesting is the use of stabilizers according to theinvention in conjunction with other stabilizers and synergism thenobserved. For example there may be included in the compositionsaccording to the invention U.V. stabilizer compounds to give high U.V.stability. Such U.V. stabilizers compounds include Tinuvin 770, CyasorbUV 531 and Tinuvin 327. [Tinuvin 327, ex Ciba-Geigy AG, is abenztriazole of the formula: ##STR6## tBu represents tertiary butyl.]Such U.V. stabilizers can be used in conjunction withC-nitroso-containing stabilizers to give compounds of high U.V.stability. Alternatively, it may be desirable if for example thermalantioxidative stability is required to use a thermal antioxidanttogether with a C-nitroso-containing stabilizer. Similarly if thestabilizer used according to the present invention is a nitrone, andU.V. stability is required it may be used in conjunction with a knownU.V. stabilizer. It has been found that when stabilizers according tothe invention are used in conjunction with for example U.V. stabilizersthere is a synergistic effect i.e. the U.V. stability of the finalcomposition is higher than might be expected on the basis of the summedactivities of the individual stabilizers when used alone. Also asynergistic effect may be observed when a thermal antioxidant is usedwith a C-nitrosamine.

The processing conditions under which the compositions according to thepresent invention are prepared are important to the obtaining ofstabilization according to the present invention. As explained above, itis believed that radicals formed mechanochemically under the high shearconditions of processing react with the stabilizers according to thepresent invention to result in the formation of a nitroxyl radical. Ifmixing is carried out under conditions which do not result in nitroxylformation the stabilization according to the invention is not obtained.Indeed observations indicate that the amount of stabilization obtaineddepends on the level of nitroxyl radical formation. Generally thecomposition according to the present invention will be prepared bysimply mixing the thermoplastic polymer and stabilizer ingredients usinga conventional extruder or injection moulding machine. The importance ofthe processing operation has been demonstrated in that the effectivenessof the present stabilizers in stabilization is dependent upon processingtime. Thus generally there is an optimum processing time to give optimalstabilization and if this time is not reached or is exceeded, reducedstabilization will be obtained.

In the commercial formulation of polymer compositions, processing timesare often fixed by the residence time of the machinery being used. Inthese circumstances it may be advantageous to produce a master-batchcomposition according to the present invention having a high content ofstabilizer. This master-batch may be obtained under processingconditions which ensure that optimum stabilization is obtained and thenmay be used as additive to a thermoplastic polymer in a conventionalextruder or injection moulding machine. In this way the optimumstabilization of the present invention may be obtained without theresidence time in the conventional extruder or conventional mouldingmachine being critical. The processing time used in the production ofthe master-batch on the other hand can be readily adjusted and is chosensuch as to provide optimum stabilization. The master-batch may suitablybe advantageous to introduce additional radical generators, for example,dialkyl peroxides, during the processing operation.

The stabilizers according to the present invention are generally used atup to 1% by weight of the thermoplastic polymer composition. Thepreferred range is usually 0.05 g to 0.5 g stabilizer per 100 g ofpolymer. In the case of master-batches however the concentration ofstabilizer will be considerably greater. For example between 2 and 15%by weight of the thermoplastic polymer composition.

The following Examples illustrate the invention.

EXAMPLE 1

Tests were carried out using the following compounds:

    ______________________________________    Structural formulae                       Code       Mp (°C.)    ______________________________________    (CH.sub.3).sub.3 CNO                       tB-NO      80-81     ##STR7##          tO-NO      63-65     ##STR8##          B-NO       --     ##STR9##          TMB-NO     158     ##STR10##         PMB-NO     157     ##STR11##         TCB-NO     142-3     ##STR12##         HB-NO      --     ##STR13##         HBB-NO     216     ##STR14##         DMA-NO     --     ##STR15##         HN-NO      103    ______________________________________     (tBu represents tertiary butyl.)

U.V. Stabilization

The C-nitroso compounds above were compounded under polymer processingconditions in a RAPRA Torque Rheometer with polypropylene at 180° C. andat the rate of 10⁻³ moles/100 g except where indicated otherwise usingthe processing times indicated below.

In each case, the composition was formed by compression moulding into afilm which was then exposed to a sun lamp black lamp (SB) whichsimulates sunlight. The time before the film became brittle wasmeasured.

The results obtained were as follows:

    ______________________________________                   Embrittlement time (hrs)                  Processing time (mins)    Nitroso compound                    7.5    10       15   20    ______________________________________     Control (no additive)                     85     85       75  --    tB--NO*         660    770      875  800    tO--NO          660    690      650  630    B--NO           330    330      280  240    PMB--NO         --     200      --   --    TCB--NO         180    200      180  160    HB--NO          --     170      --   --    HBB--NO         --     160      185  200    DMA--NO         230    260      210  190    HN--NO          --     150      --   --    ______________________________________     *1.5 × 10.sup.-3 moles/100 g.

Melt Stabilization

The melt stabilization of polypropylene obtained with C-nitrosocompounds according to the invention was measured and compared with thatof polypropylene without melt stabilizer.

Unlike polypropylene without melt stabilizer, in which the melt flowindex doubled within 5 minutes on processing in a RAPRA Torque Rheometerat 180° C., the C-nitroso compounds showed an induction period beforeany change occurred. The times taken (mins) to the end of the inductionperiod at 180° C. in a RAPRA Torque Rheometer are as follows:

    ______________________________________                   Induction period    Nitroso compound                   (mins)    ______________________________________    tB--NO         20    tO--NO         15    B--NO          25    PMB--NO        20    TCB--NO        10    HB--NO         30    HBB--NO        25    DMA-NO         30    HN--NO         10    ______________________________________

Thermal Antioxidant Activity

The C-nitroso compounds were compounded at the rate of 10⁻³ moles/100 gpolypropylene and processed at 180° C. A film was formed and the time toembrittlement (single cell air oven at 140° C.) was measured.

The results obtained were as follows:

    ______________________________________                     Embrittlement time, (hrs)                    Processing time (mins)    Nitroso compound               Colour     7.5     10    15    20    ______________________________________     Aliphatic    tB--NO     Colourless 1.0     1.0    0.5  0.5    tO--NO     Colourless 1.0     1.0    0.7  0.7    Aromatic    B--NO      Colourless --      7.0   --    --    TMB--NO    Yellow     9.0     12.0  11.0  6.5    PMB--NO    Yellow     9.5     11.5  20.5  19.0    TCB--NO    Yellow     3.5     4.5    3.5  3.0    HB--NO     Brown      --      5.0   --    --    HBB--NO    Yellow     14.0    21.0  18.5  16.0    DMA-NO     Brown      --      11.0  --    --    HN--NO     Brown      --      8.5   --    --    ______________________________________

Synergism with Phenolic Antioxidants

It can be seen from the above table that the nitroso alkanes have littlethermal antioxidant activity. However, it was found that in combinationwith a commercial hindered phenol, synergism was observed and a verythermally stable formulation was obtained. Thus Irganox 1076+tB-NO eachat 5×10⁻⁴ moles/100 g give an embrittlement time at 140° C. of >41hours. [Irganox 1076, ex Ciba-Geigy, is of the formula: ##STR16## tBurepresenting tertiary butyl.]

Nitroxyl Concentration

The concentration of nitroxyl radicals (10⁻⁵ mole/100 g) inpolypropylene after processing with the nitroso compounds in a closedmixer (180° C.) was measured. The initial concentration of C-nitrosocompound was 10⁻³ mole/100 g. The results were as follows:

    ______________________________________                   Nitroxyl concentration                   (10.sup.-5 mole/100 g)                   Processing time (mins)    Nitroso compound 10       15    ______________________________________     tB--NO          3.9      4.2    tO--NO           1.9      2.1    B--NO            4.1      4.1    PMB--NO          2.5      2.4    TMB--NO          2.3      2.1    ______________________________________

EXAMPLE 2

The following N-nitroso compounds were used

    __________________________________________________________________________    Nnitroso Compounds           Code  Mp (°C.)    __________________________________________________________________________     ##STR17##                   PS-NO 108-9     ##STR18##                   P-NO  154     ##STR19##                   DC-NO --    __________________________________________________________________________

The PS-NO was prepared as follows:

9.6 g (0.02 mole) of 2,2,6,6-tetramethyl-4-piperidinyl sebacate (Tinuvin770) was dissolved in 100 ml alcohol and cooled at 5° C. 3 ml conc. HClwas added slowly with stirring followed by the addition of sodiumnitrite solution (1.6 g in 5 ml water). The temperature was maintainedat 5° C. A cream crystalline product precipitated after removal of someof the alcohol and was recrystallized from methanol to give a productwith the correct elemental analysis (Mp 108°-109° C.).

U.V. Stabilization

The effect of processing time on the U.V. stabilizing effectiveness ofthe N-nitroso compounds was tested as in Example 1. The polymercomposition was made by mixing 10⁻³ mole of N-nitroso compound per 100 gpolypropylene using a closed mixer at 180° C. The results obtained wereas follows:

    ______________________________________                 Embrittlement time (hrs)                Processing time (min)    Antioxidant   7.5     10       15    20    ______________________________________    Tinuvin 770 (PS--H)                  --      760      --    --    PS--NO        1100    1580     1120  880    P--NO         370     570      440   330    DC--NO        200     230      190   140    No additive    85      85       75   --    ______________________________________

The effectiveness of different concentrations (10⁻⁴ g/100 g-10⁻³ g/100g) of the N-nitroso compound on the U.V. stability of polypropylene wastested. All samples were processed for 10 minutes at 180° C. The resultswere as follows:

    ______________________________________             Embrittlement time (hrs)            Concentration (× 10.sup.-4 g/100 g)    Antioxidant              1           5         10    ______________________________________     PS--NO   440         1340      1580    P--NO     155         300       570    DC--NO    135         190       230    ______________________________________

Melt Stabilization

The melt stabilization of polypropylene obtained with N-nitrosocompounds according to the invention was measured and compared with thatobtained using Tinuvin 770.

Whereas in polypropylene containing 10⁻³ mole/100 g of Tinuvin 770 themelt flow index doubled within 10 minutes of processing at 180° C. in aclosed Torque Rheometer, the corresponding bis-nitroso compound (PS-NO)had an induction period of 20 minutes before any change occurred. P-NOand DC-NO both had induction periods of 10 minutes under the sameconditions.

Nitroxyl Concentration

The concentration of nitroxyl radicals (10⁻⁵ mole/100 g) inpolypropylene after processing with the N-nitroso compounds in a closedmixer (180° C./10 minutes) was measured. The initial concentration ofN-nitroso compound was 10⁻³ mole/100 g. The results were as follows:

    ______________________________________    N--nitroso   Nitroxyl concentration    Compound     (10.sup.-5 mole/100 g)    ______________________________________    PS--NO       5.9    P--NO        4.7    DC--NO       3.6    ______________________________________

EXAMPLE 3

There were used the following nitrone compounds:

    ______________________________________    Nitrone                Code     Mp (°C.)    ______________________________________     ##STR20##             DPN      110     ##STR21##             HDPN     211     ##STR22##             MHDPN    112     ##STR23##             NDPN     176     ##STR24##             MDPN     117     ##STR25##             PBN       74     ##STR26##             HPBN     228     ##STR27##             MHPBN    178    ______________________________________     (tBu represents tertiary butyl).

U.V. Stabilization

The effect of processing on U.V. stability of polypropylene mixed withthese nitrone compounds was tested as in Example 1. The nitrones wereused at a concentration of 10⁻³ moles/100 g and the processing wascarried out in a closed mixer at 180° C. The results obtained were asfollows:

    ______________________________________                   Embrittlement time (hrs)                  Processing time (mins)    Nitrone         7.5    10       15   20    ______________________________________    Control (no additive)                     85     85       75  --    DPN             200    245      290  185    HDPN            240    210      210  200    MHDPN           140    140      140  120    NDPN            230    210      210  210    MDPN            130    150      160  140    PBN              90     90       90   90    HPBN            120    140      160  200    MHPBN           130    130      130  130    ______________________________________

Melt Stabilization

The melt stabilization of polypropylene obtained with nitrone compoundsaccording to the invention was measured and compared with that ofpolypropylene without melt stabilizer.

Unlike the melt flow index of polypropylene containing no additive,which doubled on processing at 180° C. in a closed Torque Rheometer,with the nitrones there was an induction period before any changeoccurred under the same conditions at 10⁻³ mole/100 g. The length of theinduction period is listed for typical nitrones below:

    ______________________________________                    Induction period    Nitrone Compound                    (mins)    ______________________________________    DPN             15    HDPN            >20    MHDPN           >20    NDPN            15    MDPN            15    ______________________________________

Thermal Antioxidant Activity

In addition there was tested the effect of processing conditions on thethermal antioxidant activity of the nitrones in an air oven at 140° C.The concentration of the nitrones was 10⁻³ moles/100 g. The resultsobtained were as follows:

    ______________________________________                   Embrittlement time (hrs)                  Processing time (mins)    Nitrone         7.5    10       15   20    ______________________________________    Control (no additive)                    --     0.5      --   --    DPN             1.5    1.5      1.5  1.5    HDPN            7.0    10.0     8.5  6.0    MHDPN           18.5   14.0     11.0 9.0    PBN             1.0    1.0      1.0  1.0    HPBN            3.0    4.0      2.5  2.0    MHPBN           9.0    7.5      4.5  2.5    ______________________________________

EXAMPLE 4

The synergistic effects of stabilizers used according to the presentinvention with conventional U.V. stabilizers were tested by measurementof the U.V. stabilization as in Example 1 of polypropylene compositionsprepared using such mixtures of stabilizers. By way of controlembrittlement times of polypropylene containing the stabilizers alonewere also obtained. The results were as follows:

    ______________________________________                  Concentration  Embrittlement    Additive      (× 10.sup.-4 mole/100 g)                                 time (hours)    ______________________________________    Control (no additive)                  --              85    tB--NO        5              410    Cyasorb UV 531                  5              340    Tinuvin 770   5              700    B--NO         5              210    Tinuvin 770 + tB--NO                  5 + 5          >2000    Tinuvin 770 + B--NO                  5 + 5          2000    Cyasorb UV 531 +                  5 + 5          >2000    tB--NO    ______________________________________

We claim:
 1. A process for preparing a stabilized thermoplastic polymercomposition which comprises:mixing polyethylene, polypropylene, anethylene-propylene copolymer or polyvinyl chloride with aC-nitroso-containing compound, which function as a stabilizer, underpolymer processing conditions such that nitroxyl radicals are formed. 2.The process according to claim 1, wherein said stabilizer is aC-nitroso-containing compound of formula I: wherein R represents anunsubstituted or substituted alkyl group, an unsubstituted orsubstituted phenyl or naphthyl group, or a heterocyclic aromatic orreduced heterocyclic group.
 3. The process according to claim 2, whereinsaid stabilizer is a tertiary alkyl nitroso compound.
 4. The processaccording to claim 2, wherein, in formula I, R represents a tertiarybutyl or octyl group, C(CH₂ OR')₃ or R"_(x) C(CH₂ OR')_(3-x), wherein xis 1, 2 or 3, R' represents a hydrogen atom or an unsubstituted orsubstituted alkyl or acyl group, wherein each R' substituent may be thesame or different, and R" represents an unsubstituted or alkyl-,chloro-, hydroxy-, carboxy-, cyano-, nitro- or dimethylamino-substituted phenyl or naphthyl group.
 5. The process according to claim1, wherein said stabilizer is present in an amount of up to 1% byweight.
 6. The process according to claim 1, wherein an additionalstabilizer is also mixed into the thermoplastic polymer.
 7. The processaccording to claim 6 wherein the additional stabilizer is a thermalantioxidant and/or U.V. stabilizer.
 8. The process according to claim 1,wherein a master-batch composition is initially prepared by mixing athermoplastic polymer with a nitrogen-containing stabilizer underpolymer process conditions such that nitroxyl radicals are formed, andthen mixing the master-batch composition with additional thermoplasticpolymer.
 9. A composition prepared by the process claimed in claim 1.